Vacuum cleaners generally include a motor which drives an air impeller to create a low pressure area inside of a tank or other receptacle. The tank has an inlet through which dust and debris or liquid material enter into the tank, usually from a hose. When liquid is being suctioned into the tank, it is not generally necessary to have a filter between the tank and the air impeller. For instance, in U.S. Pat. No. 4,906,265, a foam cuff 30 fits over a filter cage 20 during wet operations. When the vacuum cleaner is used on dry materials, a paper or cloth filter is placed over the foam cuff and attached with a ring. The paper or cloth filter then minimizes the amount of particulate material which escapes from the tank and passes to the air impeller. Dust or debris in the air impeller is undesirable because it may interfere with the operation of the air impeller or motor, and the dust or debris may be exhausted back out into the room.
While cloth or flat paper filters may be satisfactory, it is often desirable to use a cylindrical pleated filter for some types of vacuuming. Cylindrical or cartridge filters have a large filtering surface area and may be made of a variety of filtering materials. It has therefore been known to use a cylindrical filter with an open top and a closed bottom which is inserted over a filter cage. The top of the filter may be made of a flexible material such as rubber so that when the filter is pushed over the filter cage, the flexible material deforms and frictionally holds the filter in place.
Other systems use cylindrical filters which are open at both ends. Such filters may fit over a filter cage having a closed bottom in order to prevent material from passing around the filter and into the air impeller. Other open-ended filters may use a retainer of some type which holds the filter in place and closes the open end of the filter.
Dual filter assemblies may include two types of filter media concentrically arranged in a single filter unit, such as that disclosed in Newman, U.S. Pat. No. 5,259,854. High efficiency particle air ("HEPA") filters, which can remove 99.97% of particles larger than 0.3 microns from a stream of air, are useful for removing very small particles of dust or debris from air. Newman discloses a disposable filter cannister including an annular HEPA filter surrounded by a prefilter. There is a need for a dual filter assembly in which each of the filters can be separately removed for inspection and replacement, and in which one or both filters may be used. There is a also a need for a vacuum cleaner having a dual filter assembly in which a secondary filter, such as a HEPA filter, is protected from moisture.